1. Field of the Invention
The present invention relates to a magnetron which is used for apparatuses utilizing high frequencies, and which is intended to reduce noises.
2. Description of the Related Art
A conventional magnetron will be described with reference to drawings.
FIG. 32 is a longitudinal sectional view showing an interaction space in which electrons of a magnetron in a conventional article make motions. In this magnetron, a plurality of plate-like vanes 2 (only two vanes are shown in this drawing) are radially arranged inside an anode tube 1, and the plate-like vanes 2 are alternately connected by pressure equalizing rings 9, 10, 11, and 12. By alternately connecting the pressure equalizing rings 9, 10, 11, and 12 in this way, the magnetron will oscillate stably in a π mode. Also, a cathode 13 composed of a coiled filament 3, a pair of end hats 6 and 7, and a cathode supporting rod 8 is provided along an axial center of the anode tube 1. This filament 3 is formed of tungsten containing thorium of 1 to 2%, and is designed such that a work function is lowered and electrons are emitted easily by carburizing the surface of the filament. Furthermore, the pair of end hats 6 and 7 are arranged at both ends of the filament 3 in an axial direction in order to suppress leakage of electrons in the axial direction, and are secured to ends 3a and 3b of the filament 3. Here, since the ends 3a and 3b of the filament 3 which are secured to the end hats 6 and 7 are not carburized, they have a high work function and hardly emit electrons. Actually, an electron emission part which emits electrons is an axial free length region of the filament 3 which is carburized and is not secured to the end hats 6 and 7.
In such a magnetron, a technique of reducing the noises generated in the magnetron is suggested conventionally (for example, refer to Patent Document 1 and Patent Document 2).
FIG. 33 is a longitudinal sectional view showing a portion within an anode tube of a magnetron disclosed in the above Patent Document 1. In this magnetron, in addition to the components of FIG. 32, metallic cylindrical bodies 4 and 5 are arranged at both ends of the cathode 13. The input-side cylindrical body 4 of the cathode 13 is secured to the input-side end hat 6, and the output-side cylindrical body 5 of the cathode 13 is secured to the output-side end hat 7. Since these cylindrical bodies 4 and 5 suppress spread of the electrons emitted from the filament 3 and the magnetron is equipped with these cylindrical bodies 4 and 5, noises in a band of 30 MHZ to 200 MHz can be reduced remarkably.
In addition, FIG. 34 is a waveform chart showing noise levels of 1 GHz or less in a conventional article in which the cylindrical bodies 4 and 5 shown in FIG. 32 are not provided at all, which is actually measured by the inventors of the present application. It can be surely understood that, in the conventional article in which the cylindrical bodies are not provided at all, noises are especially high below 200 MHz, and in this respect, a reduction in noises in a band of 30 MHz to 200 MHz as described in Patent Document 1 is meaningful.
It is also is known that noises are reduced by suppressing excess electrons within an interaction space. According to a technique described in Patent Document 2, the amount of emission of electrons is suppressed and thereby noises are reduced, by setting the ratio P/d of the wire diameter d and pitch P of a filament to 2.5 or more and 3.5 or less.
Patent Document 1: JP-A4-77412
Patent Document 2: JP-A63-3417
Generally, electrons of a magnetron orbits a cathode while circling it by a force caused by an electrostatic field to which the electrons emitted from an electron emission part of the cathode are applied between the cathode and an anode, and the Lorentz force caused by a magneto-static field which is applied in the axial direction. Also, the electrons are hunted by the natural vibration of a plurality of resonators formed by plate-like vanes, an anode tube, and pressure equalizing rings, thereby forming an electron flux. Then, an induced current flows into the plate-like vanes by rotation of this electron flux, and is then converted into microwave energy by resonance of the vanes.
The shape of this electron flux depends on the intensity of a microwave electric field determined by a load combined with the magnetron, and has great influence on an oscillation frequency. Furthermore, if the intensity of the microwave electric field is strong, and the electron flux is formed into a sharp shape under the influence of the intensity, the level of noises will rise by the interaction of the crammed electrons. FIG. 36 shows noise levels when phases are changed.
It is also believed that the noises which propagate through a power line, and the noises emitted into a space are mainly generated at axial ends of an interaction space in which distortion is caused in an electric field or a magnetic field, and thus an orthogonal electromagnetic field is not maintained.
In view of the facts, in the technique disclosed in Patent Document 1, the cylindrical bodies are provided so that the electrons emitted in the axial ends of the tube cannot make motions.
Meanwhile, in the technique disclosed in Patent Document 1, noises in a band of 30 MHz to 200 MHz can be reduced, but attention is not paid to a band of 30 MHz or less in which it is difficult to suppress noises with a noise filter (not shown), composed of a coil, a capacitor, etc., which is attached to a conventional magnetron. Also, the experiments which were conducted by the inventors of the present application on the basis of the technique disclosed in Patent Document 1 show that the distribution of an electrostatic field in the interaction space may vary due to the arrangement of the cylindrical bodies 4 and 5 in the interaction space, and thus the stability of a load depending on phases tends to deteriorate significantly. Moreover, the technique disclosed in the above Patent Document 1 has a problem in that, since the cylindrical bodies 4 and 6 are secured to the end hats 6 and 7, but they are components separate from the end hats 6 and 7, respectively, the number of components is increased and the precision of assembly dimensions are not ensured easily.
Also, as shown in FIG. 37, the inventors of the present application have found out through experiments that many of noises are generated in a small current region where an anode current is about 400 mA or less. This is believed that, since the electron emission amount are set so that a peak current can be secured in, for example, non-smooth driving of half-wave voltage doubler power sources as being used for microwave ovens, electrons becomes excessive in the small current region, and consequently, noises are generated due to interaction of the excessive electrons.
Although the effect of reducing a noise of 1 MHz or less is described in the technique shown in Patent Document 2, attention is not paid to the relationship with a peak anode current value. As shown in FIG. 38, it can also be confirmed from the experiments which were conducted by the inventors of the present application on the basis of the technique disclosed in Patent Document 2 that a noise-reducing effect can be confirmed in a region where an average anode current value is 100 mA or less, but a noise-reducing effect is hardly shown in an anode current region of 200 mA and 300 mA. It is believed that this is because the electron emission amount was set so that a peak current could be ensured, and therefore electrons became excessive in the small current region.